1. Field of the Invention
The present invention relates to the field of the fabrication of flash memory cells, and more particularly to the patterning and etching of materials used in the fabrication of flash memory.
2. Discussion of Related Art
Individual features of microelectronic devices are becoming increasingly small in order to increase device density, and reduce overall device size. In particular, the dimensions of flash memory chips are being decreased. FIGS. 1a–1c illustrate different views of a flash memory array. FIG. 1a illustrates a partial overhead view of a flash memory array illustrating the placement of the source regions 110 and drain regions 120 of the floating gate transistors formed within the array and the placement of the source contacts 130 and the drain contacts 140. FIG. 1b illustrates a cross-sectional view along A—A of FIG. 1a. FIG. 2c illustrates a cross-sectional view along B—B of FIG. 1a. There are far fewer source contacts 130 than drain contacts 140. This is because there is a drain contact 140 for each drain region 120 that is shared by two floating gates 150, as illustrated in the cross-sectional view in FIG. 1b, and there is one source contact 130 that is shared among many floating gates 150. As the dimensions of the flash array are decreased, the drain contacts 140 become closer and closer together. The center-to-center distance, or “pitch” 160, between the drain contacts 140 currently cannot be etched to have a 150 nm pitch or smaller pitch due to the limitations of the resolution and depth of focus capabilities of the photolithography techniques used to pattern the drain contacts 140 as in FIG. 1c. To achieve 150 nm pitch drain contact dimensions, the width of the drain contacts 140 must be at least 80 nm or less. Currently, it is not possible using the current photolithography techniques to form contacts having a width less than 100 nm for a contact array of less than 150 pitch. The width of contacts is currently limited to approximately 100 nm by the use of 193 nm wavelength light. Moving the drain contacts 140 closer together without also decreasing the dimensions of the drain contacts 140 leads to shorts 105 between the contacts 140 as illustrated in FIG. 1d. Also, misalignment of the photoresist patterning must also be taken into account because the misalignment will cause shorts 115 between the drain contacts 140 and the first metal layer (M1) 125, as illustrated in FIG. 1e, and the misalignment may also cause more than one drain contact 140 to contact the drain region 120 as illustrated at 135 in FIG. 1e. 